Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy ink in the photo-exposed (negative working) or in the non-exposed areas (positive working) on a hydrophilic background.
In the production of common lithographic plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used.
Upon imagewise exposure of such light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
The above-described development process can suffer from the drawbacks of being relatively time-consuming and expensive. Furthermore, when volatile organic or strongly alkaline developer solutions are used, their disposal presents an environmental problem.
What is needed in the industry are photosensitive compositions which do not have the above-described drawbacks.
A variety of different materials have been used in photosensitive compositions in the past. Photosensitive compositions employing polymers with pendant acid sensitive moieties have been described in the art (Ito, H.; Ueda, M. Macromolecules 1988, 21, 1475-82). o-Nitrobenzyl substituted polyacrylates have been described in U.S. Pat. No. 3,849,137. t-Butyl substituted polyacrylates have also been reported (Ito, H.; Willson, C. G. in Proc. SPIE-Int. Soc. Opt. Eng. 1987, 771, 24; and U.S. Pat. No. 4,491,628).
U.S. Pat. No. 4,963,463 claims a radiation sensitive resin composition comprising an alkali-soluble resin, an o-quinone diazide, an acid-sensitive ester of nitrobenzyl or cyanobenzyl alcohol.
End-capped polyphthalaldehyde has been employed in imaging systems in combination with onium salts which were used as a source of photogenerated acid (Ito, H.; Willson, C. G. Polym. Eng. Sci. 1983, 23, 1013). Photoresists based on thermal polycarbonate degradation in the presence of photogenerated acid have also been described (Frechet, J. M. J.; Bouchard, F.; Houlihan, F. M.; Kryczka, B.; Eichler, E. ; Clecak, N.; Willson, C. G. J. Imag. Sci. 1986, 30, 59). Both of these systems function by cleavage of the polymer backbone.
Canadian Patent No. 672,947 describes protective films comprising copolymers of tetrahydropyran-2-yl acrylates, and glycidyl esters of acrylates. Those films are thermally processed, thereby effecting cleavage of the tetrahydropyran-2-yl groups, and subsequently causing cross-linking of the carboxylic acid and the epoxy residues.
Benzyl, benzhydryl, and triphenylmethyl acrylates have been imaged using high energy radiation (e.g. electron beam, X-ray, and ion beam) sources followed by development with aqueous alkaline solutions as described in Japanese Kokai applications 59-075244, and 58-068743.
Dimethylbenzyl methacrylates have been employed in combination with iodonium salts as deep-UV photoresists (Ito. H. Polym. Mater. Sci. Eng. 1989, 60, 142).
.alpha.-Substituted benzyl methacrylate polymers have been photoimaged and developed with an alkaline developer (Ito, H.; Ueda, M.; Ebina, M. ACS Symp. Ser. 1989, 412, 57-73).
Japanese Kokai applications 63-256492 and 63-317388 describe several direct-image lithographic plate formulations employing polymers having side-chain groups which, following an etching development step, cleave to form hydroxyl and carboxyl groups.
Japanese Kodak patent applications 53-094691 and 53-100192 describe lithographic plates containing polymers with acid labile groups including some alkoxyalkyl esters. The plates also contain cross-linking agents which appear to cross-link with carboxylic acid residues as they are formed. The plates appear to act in a negative-tone and may involve a development step.
Japanese Kokai application 62-299,313 describes the use of polymers containing acid anhydride residues in combination with o-nitrobenzyl esters of carboxylic acids. Those polymers are sensitive only to UV radiation.
U.S. Pat. No. 5,102, 771 discloses photosensitive compositions comprising : (a) a photoinitiator which generates an acid upon exposure to radiation; and (b) a polymer having acid labile groups pendant from the polymer backbone. Those materials were combined with maleic anhydride polymers to provide increased adhesion. However, the resulting photosensitive compositions have only a moderate shelf-life under ambient conditions.
U.S. Pat. No. 5,077,174 (Bauer et al.) also discloses a positive working resist composition which comprises a polymer having acid-labile pendant groups of a defined formula.
EP-A 625728 discloses a lithographic plate with an image forming layer which is UV- and IR-sensitive which can be as well positive as negative working. Said image forming layer comprises (1) a resole resin,(2) a novolac resin, (3) a latent Bronsted acid and (4) an IR absorber. By exposing with UV or IR (830 nm) light followed by a classical PS-plate development a positive working lithographic plate is obtained. If the plate is baked (60 seconds at 100.degree. C.) before the development step a negative working printing plate is obtained.
U.S. Pat. No. 4,708,925 discloses a positive working printing plate comprising a light sensitive composition comprising (1) an alkali-soluble novolac resin and an onium-salt and optionally an IR spectral sensitizing dye. By exposing with UV, visible or IR light followed by a development step with an alkali solution there is obtained a positive working printing plate. So there is still a need for acid-sensitive compounds which upon cleavage form a carboxy group instead of a phenolic group so that it is possible to develop a plate comprising an acid-sensitive composition with less aggressive developers and/or for acid-sensitive compounds which decompose at a lower temperature.